Conventionally, as a control system for internal combustion engine of this kind, one disclosed in Patent Literature 1 is known. The variable valve lift mechanism of this internal combustion engine (hereinafter referred to as “the engine”) is provided for each cylinder, and continuously changes the valve lift between a predetermined minimum value and a predetermined maximum value, and comprises a drive shaft connected to a crankshaft, and a control shaft. The drive shaft is provided with a link arm and a swing cam for driving an intake valve, and the link arm and the swing cam are connected to a rocker arm provided on the control shaft.
The control shaft extends parallel with the drive shaft, and is supported by a bearing such that it is rotatable within a predetermined angle range. Further, the control shaft has a pin protruding in a radial direction, and is connected to a rotary drive mechanism, and the bearing is formed with a protrusion. When the rotary drive mechanism causes the control shaft to rotate, the relative angles of the rocker arm on the control shaft with respect to the swing cam and the link arm are changed, whereby the valve lift is changed. Further, when the control shaft rotates in a predetermined direction, the pin on the control shaft abuts against the protrusion of the bearing, whereby the rotation of the control shaft is blocked. Further, the intake pipe of the engine is provided with a throttle valve.
In the conventional control system, normally, the throttle valve is controlled to be fully open, and according to the operating condition of the engine, the valve lift is controlled via the variable valve lift mechanism, whereby the intake air amount is controlled. Further, when the engine is in a low load operating condition, the control shaft is caused to rotate until the pin abuts against the protrusion of the bearing, and is held in the state in which the pin is in abutment with the protrusion, whereby the valve lift is held at a predetermined minimum value. By controlling the opening degree of the throttle valve, the intake air amount is controlled.
In the above-described conventional control system, in controlling the valve lift, when the engine is in the low load operating condition, the control shaft is caused to rotate until the pin abuts against the protrusion of the bearing. Therefore, there is a fear that impact occurring upon the abutment deforms the pin or the protrusion. To avoid this, it is envisaged to reduce the rotational speed of the control shaft so as to reduce the impact, or provide a cushioning member on the pin or the protrusion so as to suppress the influence of the impact. However, in the former case, time taken to rotate the control shaft until the pin abuts against the protrusion, i.e. time taken to control the valve lift to the minimum value becomes longer, which results in a longer time period taken before the intake air amount converges to an appropriate value. During the time, the operating condition of the engine becomes unstable, which can degrade drivability. On the other hand, when the cushioning member is provided on the pin or the protrusion, the minimum value of the valve lift which should be obtained when the two are in contact with each other is prone to vary, which makes it impossible to carry out accurate control of the intake air amount. Further, the addition of the cushioning member increases the manufacturing costs accordingly, and the necessity of securing a space therefor degrades the degree of freedom of design.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a control system for internal combustion engine, which is capable of reducing impact occurring when a movable part of a variable valve lift mechanism abuts against a restriction part, while maintaining excellent drivability.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. 2003-254100